The present invention relates to the manufacture of thin webs, and more particularly to the destruction of edges trims of thin webs.
The invention applies to thin webs, such as webs of PET, PEN, their copolymers, all other polymers, and non-woven or woven sheets or webs; it applies for instance to aluminum sheets or the like; such thin webs or films have a thickness less than several micrometers, e.g. 36 micrometers. Thin webs such as webs of PET, PP, PA, PS, PEN or other polymers are currently manufactured by stretching a web cast through a die in two directions. The web is stretched in a tenter oven, where it is submitted to various heating and setting steps; it is thereafter transferred to a winding machine where it is wound. In the oven, the edges of the web are clamped, and the clamps on each side of the web are moved on diverging rails, so as to carry out transversal (lateral) stretching of the web.
Thus, before the web is wound, its edges are usually trimmed. In existing manufacturing arrangements, the trimmed edges of the web may be chopped into flakes and fed to the extrusion system. A prior art chopping or grinding arrangement is represented on FIG. 1. The edge 1 trimmed from the web is fed into a vertical chute 3. and is thereafter fed into a grinder 5; in the grinder, the trim edges are cut into small pieces or flakes by rotating blades or the like, that rotate in a housing containing fixed knives. The ground edges are sucked by a fan 6, and provided to a cyclone separator 7, where the air is separated from the ground web. The ground web accumulating in the bottom of the cyclone separator may then be sucked by a fan 8, and transported to a mill 9 used for feeding the extrusion system.
Such a system could not be used for chopping thin films of a thickness of 12 micrometers or less. The necessary mechanical clearance between moving knives and fixed knives is such that the web would not be cut by the knives, but would wind around the knives, which leads to jerk of the web; the cyclone separator would have to be over-dimensioned to separate particles or flakes of thin films from the air used for their transport.
Furthermore, such existing chopping systems may create problems if the speed of the knives is higher than the speed of the web; in this case, the chopping system may pull the edge trim, and cause problems with the edge trim slitting system; this leads to uneven edges on the web roll. Finally, in existing systems, the web should be guided to the stationary knife; if the web is caught by the rotating knife, it may cause jerk in the web. Ensuring a precise transport of the web to the fixed knife is however difficult for thin films.
These problems are not encountered only with this type of device and this type of webs. More generally, there exists a need for a chopping device for all types of thin webs. in various uses, e.g. for the finishing of saleable rolls. Accordingly, there exists a need for a chopping system that may operate with thin films; such a system preferably needs to operate at high speed, that is speeds up to several hundreds of meters per minute (several meters per second) with reduced amounts of air. It should not pull the edge trims, so as not to cause any uneven edges on the wound web.
According to the invention, there is provided a chopping device for thin webs, comprising
an input section for receiving the web to be chopped;
an accelerating section for accelerating the front end of the web received from the input section, while the remainder of the web is in the input section.
Preferably, the device further comprises at least one puncturing means located between the input section and the accelerating section. The puncturing means may comprise at least one knife, with a series of needles or blades.
The accelerating section may comprise an accelerating device selected from the group comprised of Venturi effect tubes, rolls, air injection accelerating devices. It may also comprise air pulsing means for pulsing air at the puncturing means.
In one embodiment, the input section comprises air transport means.
The invention also provides a process for chopping thin webs, comprising the steps of
providing an incoming web;
accelerating the front end of the incoming web so as to tear off said front end from the remainder of the web.
The process may further comprise the step of causing the web to contact a puncturing device, and/or the step of pulsing air to cause the web to contact a puncturing device.